Mic A/B output switcher

Hello,

I am currently in the process of constructing a passive mic A/B output switcher that consists of one microphone input and two outputs. I am considering incorporating an isolation transformer on output B. Would a 10K:10K transformer be a suitable choice, or would I benefit more from using a transformer with a lower impedance?

Thanks
 
I did build this with the 10K:10K version but there is signal loss....

So what to do instead?

The transformer is driven by a 290 Ω microphone source and is loaded by 4kΩ input impedance of the microphone preamp. Which type of transformer should I use in this situation? I came across the MET-50-T transformer (http://catalog.triadmagnetics.com/Asset/MET-50-T.pdf), which has a 10K primary and 125 ohm secondary. Will this transformer effectively transfer the signal without any loss or gain?
 
I ended up with 600:600 ohm transformer. I am using a balanced test signal of 300 ohms at 100Hz and 10mV, similar to that of a microphone. This signal is routed through my switch box with A and B outputs, which then connect to a mixer with microphone preamp inputs. I am measuring the output of the mixer and have observed a -6dB loss when using a 600:600 ohm transformer on output B. Is this loss expected? Is there any way (passive solution) I can avoid this loss, and have the exact signal on both outputs. Or is this as good as it gets with passive design?